Gettin scary here...when the system is off the pressures will equalize. 134a will stand at about 74 PSI @ 72F. Running pressure would be about 26 PSI @ 30F. 404a is better for lower temps but pressures will be higher about 155 PSI @ 72F and 25 PSI @ -10F. Running pressures will vary with what sort of resriction is used but all in all a specially designed block would probably be needed. You could look into a low pressure refrigerant like 123 but that is usually used with a flooded evaporator and centrifugal compressors.
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